1. Field of the Invention
The present invention relates to a method of manufacturing a ridge-type semiconductor laser.
2. Description of the Related Art
A method of manufacturing a distributed Bragg reflector (DBR) laser diode is discussed by M. M. Raj et al. in “High-Reflectivity Semiconductor/Benzocyclobutene Bragg Reflector Mirrors for GaInAsP/InP Lasers” (Japanese Journal of Applied Physics, The Japan Society of Applied Physics, April 2001, Vol. 40, pp. 2269-2277, Part 1, No. 4A). In this method, a stacked semiconductor layer including an active layer is first formed on an entire surface of a semiconductor substrate. Subsequently, a titanium (Ti) mask for defining a semiconductor laser portion and a DBR mirror is formed by an electron beam exposure technique and a lift-off process. Subsequently, the stacked semiconductor layer is etched by using the Ti mask as an etching mask. Thus, a semiconductor laser portion and a DBR mirror are provided on the semiconductor substrate. According to this method, the semiconductor laser portion and the DBR mirror have identical stacked-semiconductor-layer structures on the semiconductor substrate.
A semiconductor integrated device in which a ridge-type semiconductor laser portion and a filter portion that includes ring resonators and so forth are integrated is discussed by T. Okamoto et al. in “Monolithic Integration of a 10 Gb/s Mach-Zehnder Modulator and a Widely Tunable Laser based on a 2-Ring Loop Filter” (2010 International Conference on Indium Phosphide and Related Materials, IEEE, 2010, pp. 390-393 (ThA1-3)). This document also concerns a method of manufacturing a semiconductor integrated device in which a stacked semiconductor layer including an active layer and provided on a semiconductor substrate is etched twice. According to this method, in a first etching step, a first region of the stacked semiconductor layer is etched to a first depth in such a manner as to form a pair of stripe grooves, whereby a ridge waveguide portion is provided. Thus, a ridge-type semiconductor laser portion is provided. Subsequently, in a second etching step, a second region of the stacked semiconductor layer adjacent to the first region is etched to a second depth larger than the first depth into a certain shape, whereby a filter portion having a mesa-shape is provided.
In the DBR semiconductor laser discussed in “High-Reflectivity Semiconductor/Benzocyclobutene Bragg Reflector Mirrors for GaInAsP/InP Lasers”, the DBR mirror includes the active layer having the same configuration as the active layer of the semiconductor laser portion. Therefore, some of light produced in the semiconductor laser is absorbed by the active layer of the DBR mirror. Accordingly, the reflectivity of the DBR mirror is reduced. Consequently, there arise problems such as a reduction in the output power of the semiconductor laser and an increase in the threshold current.
Moreover, methods of manufacturing ridge-type semiconductor lasers are particularly complicated, and it is difficult to accurately align the semiconductor laser portion and the DBR mirror. Specifically, such a method of manufacturing a semiconductor laser includes, for example, a step of forming a stacked semiconductor layer on an entire surface of a semiconductor substrate, a step of forming a ridge waveguide portion by etching a region of the stacked semiconductor layer that is to become the semiconductor laser portion, and a step of forming a DBR mirror having a periodical structure by etching another region of the stacked semiconductor layer that is to become the DBR mirror.
The ridge waveguide portion needs to be provided in a region above the active layer of the semiconductor laser portion. On the other hand, the DBR mirror needs to be provided in a region including a part at the same level as the active layer of the semiconductor laser portion to reflect light emitting from the active layer. Therefore, the etching depth of the stacked semiconductor layer required in the step of forming the ridge waveguide portion differs from the etching depth of the stacked semiconductor layer required in the step of forming the DBR mirror. Hence, it is difficult to perform these steps simultaneously in one etching step.
Accordingly, the step of forming the ridge waveguide portion and the step of forming the DBR mirror tend to be performed in two separate etching steps, similarly to the method of manufacturing a semiconductor integrated device including a ridge-type semiconductor laser portion and a filter portion that includes ring resonators and so forth discussed in “Monolithic Integration of a 10 Gb/s Mach-Zehnder Modulator and a Widely Tunable Laser based on a 2-Ring Loop Filter”. Consequently, the method becomes complicated. Moreover, misalignment between the mask used in the first etching step and the mask used in the second etching step is often occurred. Consequently, it is difficult to accurately align the ridge-type semiconductor laser portion and the DBR mirror.